The present invention relates to a ground fault interrupter for a frequency converter. The present invention also relates to a method for operating a ground fault interrupter for a frequency converter
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
An electrical machine can, for example, be operated on an electrical power supply network by means of a frequency converter. For this purpose, AC voltages are generated by the frequency converter in windings of the machine, the frequency and effective value of which are predetermined by the frequency converter. In such cases the frequency converter first charges up a smoothing capacitor in a DC link circuit with mains power by means of a rectifier, which it receives via the mains lines of the ground fault interrupter. Thus a DC voltage is made available in the DC link circuit. An inverter of the frequency converter then creates from the DC voltage the individual AC voltages in the windings of the electrical machine. The frequency and the average amplitude of the AC voltages can be set here for example based on a pulse width modulation of switching signals for semiconductor power switches, with the aid of which the AC voltages are created in the inverter.
However, operational leakage currents, i.e. currents which do not flow in and out exclusively via the power lines of the ground fault interrupter, can occur because of pulse width modulation. Instead the circuit is closed via a ground or chassis potential. The leakage currents are made possible in this case by a capacitive coupling between the frequency converter or the electrical machine on the one hand and objects its environment on the other hand.
A leakage current represents a common mode current in the power lines. It thus corresponds in relation to the flux direction of the current to a residual current which likewise flows as a common mode current by ground or chassis potential if for example insulation of the windings in the motor is damaged and therefore a current flows away out of the frequency converter via a housing of the motor.
If an arrangement of frequency converter and electrical machine for protecting personnel and/or preventing a fire is thus operated via a conventional ground fault interrupter, the problem can arise of such a power switch not being able to distinguish between an actual residual current (caused by a damaged cable for example) and an operationally-generated leakage current. The result is therefore undesired faulty tripping of the ground fault interrupter.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved ground fault interrupter to be used for a frequency converter.